Does hot water freeze faster than cold?

Betsy's new boss is coming to dinner, and she wants to make a good impression. There's a lot to do, so her husband Billy decides to lend a hand in the kitchen. As he fills ice trays from the cold water tap, Betsy remarks, "You should put hot water in the ice trays because it freezes quicker." Without thinking, Billy obeys, then starts to fill the kettle with hot water. With a growing impatience, Betsy says "You should fill the kettle with cold water because it boils faster." Billy obeys again, but this time he can't help but wonder if his wife has been somewhat impaired by the extra stress of entertaining. While Billy retreats to safer ground in the living room, let's get an SwRI Whizard to review Betsy's logic.

"Betsy is perfectly right -- she just happens to be right about the wrong thing. First, let's consider the ice trays. As the ice tray is placed in the freezer, the water has a certain amount of energy. The hotter the water, the more thermal energy it contains. The job of the refrigerator is to remove the energy from the water until it cools to the freezing point and then transforms into solid ice. The actual freezing process depends on the cooling rate and the extension of crystals from the ice tray walls into the center of each cube. Depending on pressure, temperature, and the way the ice is further cooled, there can be as many as nine types of ice. While all of this is extremely interesting, it doesn't have much to do with Billy's problem of whether hot or cold water in the tray reaches freezing point first.

"The most convenient way to estimate the rate at which water is cooled is to use a method similar to Newton's Law of Cooling, which many of us learned about in high school science class. Newton's Law states that the cooling rate of an object is the product of the heat transfer coefficient and the temperature difference between that object and the surrounding environment.

"Except in the simplest situations, heat transfer coefficients are notoriously hard to estimate accurately. Flow velocities, surface shape and texture, and fluid properties all affect the value of the heat transfer coefficient. An ice tray in a freezer sounds like a simple situation, but the odd shapes, unknown air flow by fans and natural convection, and surrounding objects make this a very complicated flow problem. In the case here, there will be very little difference in the heat transfer coefficients for the cold and hot water as long as the trays are placed in roughly the same location in the freezer.

"The temperature difference between the water and the surrounding air in the refrigerator is the more dominant factor for the situation here. When the water temperature is higher, there is a larger temperature difference between it and the surrounding air. This results in the hotter water cooling faster than the cooler water. As the tray cools, however, the temperature difference between the water and the freezer air decreases, which means that as the water cools, the rate at which it cools also decreases. The cold water is also cooling and because it has a head start, it manages to always stay ahead of the warmer water in the race to freezing point. The hot water may initially cool faster than the cold water, but the cold water will freeze sooner than the hot water.

"The same sort of thing happens in reverse in the water kettle. We still have to worry about a heat transfer coefficient between the water and the bottom of the kettle, and there is now a larger temperature difference between the cold water and the bottom of the kettle than between the hot water and the kettle. In this case, the cold water will heat at a faster rate, but the hot water manages to get to the boiling point sooner than the initially cold water.

"For now, it sounds as if Billy better stay out of Betsy's way. Maybe while he is cleaning up later, Billy can use this faster/sooner approach for setting things right."

Thanks to this month's Whizard, Steve Green, a principal engineer in the Fluids Engineering Department of the Mechanical and Materials Engineering Division. Green specializes in fluid mechanics and heat transfer research.

The Lighter Side SwRI Home

March 25, 2013